A lithium ion secondary battery is lighter in weight and has higher capacity than a nickel-cadmium battery, a nickel hydrogen battery, and the like. For this reason, the lithium ion secondary batteries have been widely used as a power supply for mobile electronic appliances. The lithium ion secondary battery is also a strong candidate for a power supply to be mounted on hybrid automobiles and electric automobiles. With the size reduction and higher sophistication of the mobile electronic appliances in recent years, the lithium ion secondary battery used for the power supply is expected to have higher capacity.
In view of the above, an alloy-based negative electrode material including silicon and silicon oxide has attracted attention. Silicon can electrochemically intercalate and deintercalate lithium ions. Silicon enables the charging and discharging with much higher capacity than graphite. In particular, it is known that silicon exhibits a theoretical discharging capacity of 4,210 mAh/g, which is 11 times as high as that of graphite.
However, when silicon or a silicon compound is used as a negative electrode active material, the expansion and contraction of an electrode caused by the intercalation and deintercalation of lithium ions along with the charging and discharging are remarkably larger than those in the case of using graphite as a negative electrode active material. Therefore, in the lithium ion secondary battery using an alloy-based negative electrode material of silicon or the like as the negative electrode active material, the negative electrode active material layer expands or contracts due to the repetition of the charging and discharging. This applies a large stress on the negative electrode.
This may result in problems that the crack occurs in the negative electrode active material layer formed on a negative electrode current collector, and the negative electrode active material layer and the negative electrode current collector are separated. As a result, the conductive path is blocked between the negative electrode active material and the negative electrode active material and between the negative electrode active material and the negative electrode current collector. This leads to the lower cycle characteristic of the lithium ion secondary battery.
In view of the above problem, Patent Literature 1 suggests to use the polyacrylic resin, which has a predetermined mechanical characteristic, as the negative electrode binder in order to improve the adhesion between the negative electrode active material layer and the negative electrode current collector and to suppress the volume expansion of the negative electrode. Since the polyacrylic resin can use water as the solvent, the polyacrylic resin has advantages that the environmental burden in the fabrication is small and the cost can be suppressed.
In general, as the adhesion between the negative electrode active material layer and the negative electrode current collector is higher, the falloff of the negative electrode active material layer in the charging/discharging cycle is small. The deterioration is therefore suppressed. For this reason, the resin with higher adhesion is more preferable. However, too high adhesion between the polyacrylic acid and the negative electrode current collector is a problem.
In the negative electrode including the negative electrode active material with the large expansion and contraction such as silicon, too high adhesion between the binder and the negative electrode current collector causes the stress from the expansion and contraction of the negative electrode active material to be applied to the negative electrode current collector in the quick charging/discharging. This produces the irreversible form change of the negative electrode, i.e., a crease.
The quick charging/discharging in this specification refers to the charging and discharging at a current density of 10 C or more. Note that 1 C is the current value at which, when a battery cell with a nominal capacity is charged at a constant current, the charging is completed in an hour.